EditorLink.available_exams: cope with JSON decode error

Author: christianp

numbas_lti/models.py

@@ -1939,7 +1939,10 @@ def available_exams(self):
             from . import tasks
             tasks.editorlink_update_cache(self)
         if self.cached_available_exams:
-            return json.loads(self.cached_available_exams)
+            try:
+                return json.loads(self.cached_available_exams)
+            except json.JSONDecodeError:
+                return []
         else:
             return []